1. Field of the Invention
The invention relates to a lower side bearing of a wheel truck for supporting load of a freight car body, and more particularly to a lower side bearing for supporting load in empty and heavy states.
2. Description of the Related Art
As a critical part of a railroad freight car, a typical wheel truck includes two side frame assemblies and a bolster assembly. Journal-box guides disposed on two ends of the frame assembly are fixed on a front wheel pair and a rear wheel pair via roller bearing adapters, respectively. The bolster assembly has two ends, each of which is mounted in a central square box of the side frame assembly via a spring suspension device for supporting the load from the bolster assembly. The bolster assembly includes a lower center plate in the center and two lower side bearings on two ends. The lower center plate and the lower side bearings are matched with an upper center plate and two upper side bearings on the lower base of the freight car for supporting the weight of the freight car.
In early supporting structure of the freight car, the lower center plate of the bolster assembly supported all loads of the car body, whereas the lower side bearings assisted for positioning. Thereafter, in order to improve the critical speed of an empty freight car, the lower center plate was improved as a primary bearing structure, and the lower side bearing was improved to assist for supporting. The friction between the upper and lower side bearings can act as a resistance during the turnaround of the wheel truck to meet the requirement of speed-raising.
In above descriptions, the supporting achieved totally by the lower center plate and the supporting achieved by the combination of the lower center plate and the lower side bearings are commonly called center plate-type supporting. The wheel truck having the center plate-type supporting is advantageous in that when crossing curved tracks, the wheel truck is flexible in turning around, and the load is uniformly distributed on the wheels. However, it has defects that the vertical load of the body is directly applied on the center of the bolster assembly, and transmitted to the square boxes via the bolster assembly, which results in a large bending moment and sectional area of the bolster assembly. Correspondingly, the weight and the production cost of the assembly are increased, and the center plate has a low stability in rolling.
To solve the above problems, two lower side bearings are employed to replace the center plate to support all loads from the car body. However, it is very difficult to design a bearing structure that meets the supporting requirements for both a heavy loaded state and an empty loaded state. During the heavy loaded state, the friction torque of the lower side bearing is too large, and the horizontal force on the curved wheeltrack from the car body increases, thereby increasing the possibility of digression and the damage on the rim. To lower the friction torque in the heavy loaded state, decreasing the friction coefficient of the lower side bearings is practicable, which, however, will result in a too small friction torque in the empty loaded state, and thus, the critical speed of the wheel truck will be lowered during the empty loaded state. Therefore, developing a side bearing structure that can work well in both empty loaded state and heavy loaded state has actual significance for lowering the car body and improving the running performance of the freight car.